A conventional outboard motor is a self-contained unit that can be fitted on the transom of a boat and that includes an engine, transmission and propeller (or jet drive). The entire unit can pivot relative to the transom about a vertical steering axis, to control the direction of thrust from the propeller—and thus steer the boat. The entire unit can also be pivoted relative to the transom about a transverse, horizontal trim/tilt axis, to trim the angle of attack of the thrust and/or to tilt the unit up, e.g. when not in use.
The conventional configuration of an outboard motor includes an engine in an upper part (power head)—typically with a vertical crank shaft, although horizontal crank shafts have also been used. A drive shaft extends vertically from the motor in a mid-section that also typically houses an exhaust. A lower unit houses a gearbox, where power is transmitted from the vertical drive shaft to a horizontal propeller shaft. The power head, mid-section and lower unit are attached together to form a single unit that pivots about the steer axis and trim/tilt axis, as described above.
The configurations of these motors, which include attachments to the boat's transom that allows the entire motor to pivot about its steering axis and about its trim/tilt axis is complicated—partly due to the multiplicity of pivot axes and partly because the entire engine needs to pivot about these axes—which can require large forces in the case of larger motors and which requires adequate space for the entire unit to pivot. In order to accommodate these pivotal movements, the units are usually supported well aft of the transom, but the distance between the unit and the transom provides a moment arm and increases forces on the transom. The forces required to pivot these units, as well as the forces exerted on the transom, limit the use of outboard motors to relatively small motors.
In many cases, stricter limitations on exhaust emissions are applied to inboard motors than to outboard motors and compliance with emissions limitations increase manufacturing costs—resulting in cost benefits from using outboard motors. However, only smaller engines have conventionally been used in outboard configurations and the use of larger engines in outboard motors tends to be too complex, cumbersome and/or costly.
The most common design for the sterns of modern leisure power boats includes a planar transom that is either vertically orientated or is very steeply inclined (“raked”—i.e. angled aft with a small “transom angle” relative to vertical). If a particular motor configuration requires deviation from a standard stern design offered by a hull manufacturer, the motor configuration can only be used if the hull manufacturer offers an alternative stern design (which increases tooling and/or manufacturing costs) or a standard hull needs to be modified after manufacture (also at considerable cost and/or detriment to hull quality). Accordingly, there is significant resistance to marine motor configurations that require deviation from conventional, standard transom designs.
A stern drive has been disclosed in WO2012/168767, which uses a drive configuration that is simple and compact and can accommodate engines in various space-saving configurations, but the drive uses an inboard motor, which requires adherence to strict emissions limitations. Further, the stern drive requires a non-standard transom angle of about 45 degrees. The stern drive disclosed in WO2012/168767 holds benefits in handling and performance resulting from an inclined steering axis.
The present invention seeks to provide a marine propulsion system that uses an outboard motor, is relatively simple and cost effective, can be fitted on a conventional transom, can use a relatively large motor, makes effective use of space and provides good handling and performance.